Objective lens for endoscope

ABSTRACT

An objective lens for an endoscope is provided and has a lens group A and a lens group B that are movable in a direction of an optical axis. In the objective lens, first focal adjustment for observing a point between a most-distant point and an intermediate point is performed by moving the lens group A from a lens arrangement for observing the most-distant point; and second focal adjustment for observing a point between the intermediate point to a nearest point is performed by moving the lens group B from a lens arrangement for observing the intermediate point.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an objective lens to be provided at atip of an endoscope, and more particularly to an objective lens for anendoscope (hereinafter, sometimes referred to as “endoscopic objectivelens”) capable of performing focal adjustment (hereinafter, sometimesreferred to as “focusing”), by moving part of the system lenses, instates from a state suited for observing an object in its entirety(hereinafter, referred to as “distant-side observation state) to a statesuited for observing, with magnification, a part of the object(hereinafter, referred to as “a nearside magnifying observation state”).

2. Description of Related Art

As for the endoscopic objective lenses of this kind, there are knownthose described in Japanese Patent No. 2876252 and JP-A-2001-91832.

The endoscopic objective lens, described in Japanese Patent No. 2876252,is arranged with four groups whose refractive powers are negative,positive, negative and positive in order from the object side. By movingthe third group along the optical axis, focal adjustment can beperformed from a distant-side observation state to a nearside magnifyingobservation state. By virtue of the lens groups movable in position,observation is available not only at the both ends for the most-distantand nearest points but also at the intermediate region between the bothends.

Meanwhile, the endoscopic objective lens, described in JP-A-2001-91832,is arranged with four groups whose refractive powers are negative,positive, negative and positive in order from the object side. By movingthe second and third groups or the third and fourth groups along theoptical axis, focal adjustment can be performed from a distant-sideobservation state to a nearside magnifying observation state.Furthermore, the magnification of the lens in use can be desirablychanged in the intermediate region between the both ends for themost-distant and nearest points.

However, in the endoscopic objective lenses described in Japanese PatentNo. 2876252 and JP-A-2001-91832, the observing magnification greatlychanges upon focusing in the nearside magnifying observation state.Thus, the subject to be observed (i.e., the object) is ready to beplaced out of the field of view. For the endoscopic objective lens, itis considerably difficult to perform focal adjustment where theobserving magnification changes greatly, because the depth ofobservation (the depth of field) is narrow in the nearside magnifyingobservation state.

SUMMARY OF THE INVENTION

An object of an illustrative, non-limiting embodiment of the inventionis to provide an endoscopic objective lens capable of easily performingfocal adjustment with a small change of observation magnification uponperforming the focal adjustment in the nearside magnifying observationstate.

According to one aspect of the invention, a focal adjustment at a pointbetween the most-distant point and an intermediate point and a focaladjustment at a point between the intermediate point and the nearestpoint are performed by means of respective lenses different from eachother.

Namely, an endoscopic objective lens according to one aspect of theinvention includes: a lens group A and a lens group B that are movablein a direction of an optical axis, wherein first focal adjustment forobserving a point between a most-distant point and an intermediate pointis performed by moving the lens group A from a lens arrangement forobserving the most-distant point; and second focal adjustment forobserving a point between the intermediate point to a nearest point isperformed by moving the lens group B from a lens arrangement forobserving the intermediate point.

In one aspect of the invention, the endoscopic objective lens preferablysatisfies conditional expressions (1) and (2) given below:1.2<f _(M) /f _(F)  (1)0.9<|f _(N) /f _(M)|<1.1  (2)where

f_(M): focal length of the endoscopic objective lens (the entire system)in observing the intermediate point,

f_(F): focal length of the endoscopic objective lens (the entire system)in observing the most-distant point, and

f_(N): focal length of the endoscopic objective lens (the entire system)in observing the nearest point.

A lens constituting the lens group A and a lens constituting the lensgroup B may not overlap each other. In this case, the lens group B canbe constituted by one cemented lens.

Meanwhile, the lens group A can be constituted by one group. On theother hand, the lens group A can be arranged with two groups that are alens group having a positive refractive power and a lens group having anegative refractive power so that the two groups can take respectivemoving paths different from each other during the first focaladjustment.

Meanwhile, in one aspect of the invention, the endoscopic objective lenscan further includes a lens group C in a position closest to an object,the lens group C being fixed during the first and second focaladjustments, wherein the objective lens satisfies conditionalexpressions (3) to (5) given below:1.2<f _(M) /f _(F)<2.5  (3)4.0<DF/f _(F)<15.0  (4)2.0<|β_(CN)/β_(CF)|<8.0  (5)where

D_(F): a length of the endoscopic objective lens (the entire system) inobserving the most-distant point (geometric distance from an object-sidesurface of the lens arranged closest to the object to an image-sidesurface of the lens arranged closest to the image side),

β_(CN): a magnification of the lens group C in observing the nearestpoint, and

β_(CF): a magnification of the lens group C in observing themost-distant point.

Meanwhile, in one aspect of the invention, the endoscopic objective lenscan include: a first lens group having a negative refractive power; asecond lens group having a positive refractive power; a third lens grouphaving a negative refractive power; a fourth lens group having apositive refractive power; and a fifth lens group having a positiverefractive power, that are arranged in order from the object side,wherein the lens group A is constituted by the third lens group, and thelens group B is constituted by the fifth lens group.

Meanwhile, in one aspect of the invention, the endoscopic objective lenscan include: a first lens group having a negative refractive power; asecond lens group having a positive refractive power; a third lens grouphaving a negative refractive power; a fourth lens group having apositive refractive power; and a fifth lens group having a positiverefractive power, that are arranged in order from the object side,wherein the lens group A is constituted by the second and third lensgroups, and the lens group B is constituted by the fifth lens group.

Meanwhile, preferably, an auto-focus mechanism is further provided forautomating the second focal adjustment.

Note that “the most-distant point” means the most distant point amongpoints in a distance from the objective lens to an object, while “thenearest point” means the nearest point thereof. Meanwhile, “theintermediate point” signifies a point located between the most-distantpoint and the nearest point wherein not necessarily meant a center pointof between the most-distant point and the nearest point.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention will appear more fully upon considerationof the exemplary embodiment of the invention, which are schematicallyset forth in the drawings, in which:

FIG. 1 is a view showing an endoscopic objective lens according to anexemplary example 1 of the present invention;

FIG. 2 is a view showing an endoscopic objective lens according to anexemplary example 2 of the present invention;

FIG. 3 is a view showing an endoscopic objective lens according to anexemplary example 4 of the present invention;

FIG. 4 shows aberration diagrams of an endoscopic objective lensaccording to an exemplary example 1 of the present invention;

FIG. 5 shows aberration diagrams of an endoscopic objective lensaccording to an exemplary example 2 of the present invention;

FIG. 6 shows aberration diagrams of an endoscopic objective lensaccording to an exemplary example 3 of the present invention; and

FIG. 7 shows aberration diagrams of an endoscopic objective lensaccording to an exemplary example 4 of the present invention.

DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS

Although the invention will be described below with reference to theexemplary embodiments thereof, the following exemplary embodiments andmodifications do not restrict the invention.

According to an exemplary embodiment, the first focal adjustment, for anobservation point in a range from a most-distant point to anintermediate point, is performed by moving the lens group A from a lensarrangement of observing the most-distant point, while second focaladjustment, for an observation point in a range from the intermediatepoint to a nearest point, is performed by moving the lens group B from alens arrangement of observing the intermediate point. This makes itpossible to reduce the change of observing magnification duringperforming a focal adjustment in the nearside magnifying observationstate, thus facilitating the focal adjustment.

With using the drawings, explanation is now made on an endoscopicobjective lens according to two exemplary embodiments of the presentinvention.

With FIG. 1, explanation is first made on an endoscopic objective lensaccording a first exemplary embodiment. FIG. 1 shows a basic arrangementof an endoscopic objective lens according to an example 1 of theinvention. Incidentally, in FIG. 1, the optical axis is shown by aone-dot chain straight line extending in a left-right direction.Meanwhile, in FIG. 1, the most-distant observation state is shown with alens arrangement for observing the most distant point (focal adjustmentis for the most distant point). The intermediate observation staterefers to a lens arrangement to observe the intermediate point (focaladjustment is for the intermediate point), while the nearest observationstate refers to a lens arrangement to observe the nearest point (focaladjustment is for the nearest point). This is the same as for those ofFIGS. 2 and 3 respectively showing the lens basic arrangements ofendoscopic objective lenses in example 2 and 4.

As shown in FIG. 1, an endoscopic objective lens includes, in order fromthe object side, a first lens group G₁ having a negative refractivepower and being fixed, a second lens group G₂ having a positiverefractive power and being movable along the optical axis, a third lensgroup G₃ having a negative refractive power and being movable along theoptical axis, a fourth lens group G₄ having a positive refractive powerand being fixed, and a fifth lens group G₅ having a positive refractivepower and being movable along the optical axis.

In this endoscopic objective lens, a lens group A (G_(A)) is constitutedby two lens groups of the second and third lens groups G₂, G₃. Where anobject lies between the most-distant point and the intermediate point,first focal adjustment can be performed by simultaneously moving thesecond lens group G₂ toward the object along the optical axis and thethird lens group G₃ toward the image along the optical axis from therespective positions in the most-distant observation state shown in theupper in FIG. 1 such that the respective moving paths are different fromeach other.

In this endoscopic objective lens, a lens group B (G_(B)) is constitutedby only the fifth lens group G₅. In the near-side magnifying observationstate in which the observation point is in a range of from anintermediate point to the nearest point, second focal adjustment isperformed by moving the fifth lens group G₅ toward the object along theoptical axis from the intermediately observation state shown in themiddle of FIG. 1.

Incidentally, the distant-side observation state means a state suitedfor an observation in which the distance to the object is great, theangle of view is wide and the range is broad. Meanwhile, the nearsideobservation state means a state suited for an observation in which thedistance to the object is small and magnification is partially done.Meanwhile, the most-distant observation state is a state in which themagnification on the entire system is rendered the smallest in thedistant-side observation state. On the other hand, the nearestobservation state is a state in which the magnification on the entiresystem is rendered the greatest in the nearside magnifying observationstate. This is the same as for those of FIGS. 2 and 3 showing the basicarrangements of endoscopic objective lenses of examples 2 and 4.

Between the third and fourth lens groups G₃ and G₄ of the endoscopicobjective lens, an aperture stop 1 is arranged to move together with thethird lens group G₃ during the first focal adjustment. Meanwhile, thereare arranged a path-change prism 2 and a cover glass 3, on the imageside of the fifth lens group G₅. On the image side of the cover glass 3,a CCD device and an image-guide fiber, not shown, are arranged to coveyimage information. In FIG. 1, the aperture stop 1 is arranged contactedon the object side of the third lens group G₃. During first focaladjustment, it can be moved together with the third lens group G₃ towardthe image side.

In the endoscopic objective lens, a lens group C (G_(C)) is arrangedclosest to the object and formed by the first lens group that is fixedduring first and second focal adjustment, thus satisfying conditionalexpressions (1), (2), (4) and (5) below.1.2<f _(M) /f _(F)  (1)0.9<|f _(N) /f _(M)|<1.1  (2)4.0<D _(F) /f _(F)<15.0  (4)2.0<β_(CN)/β_(CF)<8.0  (5)where

f_(M): a focal length of the endoscopic objective lens in observing anintermediate point,

f_(F): a focal length of the endoscopic objective lens in observing themost distant point,

f_(N): a focal length of the endoscopic objective lens in observing thenearest point,

D_(f): a focal length of the endoscopic objective lens in observing themost distant point (geometric distance of from the object-side surfaceof the lens positioned closest to the object to the image-side surfaceof the lens positioned closest to the object),

β_(CN): a magnifying power of the lens group C in observing the nearestpoint, and

β_(CF): a magnifying power of the lens group C in observing the mostdistant point.

The conditional expression (1) preferably defines the upper limit as inthe conditional expression (3) below, and hence provided to satisfy theconditional expression (3) for the FIG. 1 endoscopic objective lens.1.2<f _(M) /f _(F)<2.5  (3)

With FIG. 3, explanation is now made on an endoscopic objective lensaccording to a second exemplary embodiment of the present invention.FIG. 3 shows a basic arrangement of an endoscopic objective lensaccording to an exemplary example 4 of the invention.

As shown in FIG. 3, the endoscopic objective lens includes, in orderfrom the object side, a first lens group G₁ having a negative refractivepower and being fixed, a second lens group G₂ having a positiverefractive power and being fixed, a third lens group G₃ having anegative refractive power and movable along the optical axis, a fourthlens group G₄ having a positive refractive power and being fixed, and afifth lens group G₅ having a positive refractive power and movable alongthe optical axis.

In this endoscopic objective lens, a lens group A (G_(A)) is constitutedby only the third lens groups G₃. The first focal adjustment in adistant-side observation state is performed by moving the third lensgroup G₃ toward the object along the optical axis from a position in themost-distant observation state shown in the upper in FIG. 3.

Meanwhile, in this endoscopic objective lens, a lens group B (G_(B)) isconstituted by only the fifth lens group G₅. The second focal adjustmentin a nearside magnifying observation state is performed by moving thefifth lens group G₅ toward the object along the optical axis from aposition in the intermediately-observation state shown in the middle inFIG. 3.

Furthermore, the endoscopic objective lens is provided with an aperturestop 1 fixed between the second lens group G₂ and the third lens groupsG₂, G₃. Meanwhile, a path-change prism 2 and a cover glass 3 arearranged on the image side of the fifth lens group G₅, similarly to thefirst embodiment. In a position closer to the image with respect to thecover glass 3, a CCD device and an image-guide fiber, not shown, arearranged to covey image information.

Meanwhile, in this endoscopic objective lens, a lens group C (G_(C)) isarranged in a position closest to the object, which is formed by thefirst and second lens groups G₁, G₂ that are fixed during the first andsecond focal adjustment, thus satisfying the conditional expressions (1)to (5) similarly to the first embodiment.

With the endoscopic objective lens structured like the first or secondembodiment, the observing magnification can be reduced in change duringthe focal adjustment in a nearside magnifying observation state. Thus,focal adjustment is made easy to perform.

The conditional expression (1) defines the magnification change of theentire system caused by changing the focal length in the first focaladjustment. In the case lower than the lower limit thereof, when toobtain the same magnification, observation comes on a region where isnot well illuminated with the light from the light guide. Meanwhile, itleads to a lowered magnification for the intermediate point.

The conditional expression (2) defines the magnification change of theentire system caused by changing the focal length in the second focaladjustment. In the case set up outside the range thereof, the observingmagnification greatly changes during focal adjustment. This makes focaladjustment difficult to perform because the object is readily put out ofthe field of view.

The conditional expression (3) defines the upper-limit value notestablished in the conditional expression (1). In the case of exceedingthe upper-limit value, the lenses have an increased amount of movementthus leading to a size increase of the entire lens system and hence anincreased length at the tip of the endoscope. This results in anincreased pain that the patient is to suffer or a difficult manipulationof the endoscope.

The conditional expression (4) defines the ratio of the length of theentire lens system in a most-distant observation state to the length ofthe entire lens system in the most-distant observation state. In thecase of exceeding the upper-limit value, the endoscope increases in itstip thus resulting in an increased pain that the patient is to suffer ora difficult manipulation of the endoscope. Meanwhile, in the case ofbelow the lower limit thereof, there is obtained a less space for thelens group to move during focal adjustment, thus delimiting to a narrowrange the position where focal adjustment is available in observation.

The conditional expression (5) defines the ratio of the magnification ofthe lens group C (G_(C)) in the nearest observation state to that in themost-distant observation state. For the lens generally termed a zoomlens, the value corresponding to the conditional expression (5) results1. For the endoscope according to one aspect of the invention, whenshifted from the most-distant observation state into the nearestobservation state, the distance to the object decreases as the nearestobservation state is neared. This shows that the magnification ischanged by utilization of the action that the object looks magnified. Inthe case of below the lower-limit value, the magnification isinsufficient in the nearest observation state. Meanwhile, in the case ofexceeding the upper limit thereof, the distance to the object isexcessively near in the nearest observation state, thus resulting in anobservation on a region where is not well illuminated with the lightfrom the light guide.

Examples 1 to 4 of the invention will be explained in detail in thefollowing.

Example 1

FIG. 1 shows a basic arrangement of an endoscopic object lens accordingto an example 1, in the states of most-distant observation, intermediateobservation and nearest observation.

The endoscopic objective lens of the example 1 includes, in order fromthe object side, a first lens group G₁ having a negative refractivepower and being fixed, a second lens group G₂ having a positiverefractive power and movable along the optical axis, a third lens groupG₃ having a negative refractive power and movable along the opticalaxis, a fourth lens group G₄ having a positive refractive power andbeing fixed, and a fifth lens group G₅ having a positive refractivepower and movable along the optical axis, as explained in the firstembodiment.

The first lens group G₁ is constituted by a first lens L₁ formed as aplane-concave lens whose concave surface is directed toward the imageside, a second lens L₂ formed as a double-concave lens having an intensecurvature in its image-side surface as compared to that in theobject-side surface, and a third lens L₃ formed as a double-convex lens.The second lens L₂ and the third lens L₃ are joined together. As wasexplained in the first embodiment, a lens group C (G_(C)) in the example1 is constituted by the first lens group G₁.

The second lens group G₂ is constituted by the one, fourth lens L₄formed as double-convex lens while the third lens group G₃ is by theone, fifth lens L5 formed as a plane-concave lens whose concave surfaceis directed toward the image side. Meanwhile, in the example 1, a lensgroup A (G_(A)) is constituted by two groups, i.e. the second lens groupG₂ and the third lens group G₃, as explained in the first embodiment.The first focal adjustment, in the most-distant observation state, isperformed by simultaneously moving the second lens group G₂ (the fourthlens L₄) toward the object side and the third lens group G₃ (the fifthlend L₅) toward the image side along the optical axis from therespective positions in the most-distant observation state shown in theupper in FIG. 1 in a manner such that the respective moving paths aredifferent from each other (no movement is made during the second focaladjustment).

In example 1, an aperture stop is provided in contact with theobject-side surface of the fifth lens L₅ on the optical axis. Theaperture stop 1 is structured to move together with the fifth lens L₅during the first focal adjustment (no movement is made during secondfocal adjustment).

The fourth lens group G₄ is constituted by a sixth lens L₆ formed as adouble-convex lens having an intense curvature in the object-sidesurface as compared to that in the image-side surface and a seventh lensL₇ formed as a negative meniscus lens whose concave surface is directedtoward the image side.

The fifth lens group G₅ is constituted by one cemented lens that is madeby joining together are an eighth lens L₈ formed by double-convex lensand a seventh lens L₇ formed by a negative meniscus lens whose concavesurface is directed to the object sode. Meanwhile, as explained in thefirst embodiment, in the example 1, a lens group B (G_(B)) isconstituted by only the fifth lens group G₅. The second focaladjustment, in a nearside magnifying observation state, is performed bymoving the fifth lens group G₅ along the optical axis from a position inthe intermediate observation state shown in the middle in FIG. 1 (nomovement is made during the first focal adjustment).

Incidentally, the second focal adjustment can be structurally done bymanually moving the fifth lens group G₅. However, it is possible toprovide an auto-focus mechanism for automating the second focaladjustment. Such an auto-focus mechanism can be structured, forembodiment, by a drive mechanism for moving the fifth lens group G₅along the optical axis and control means for controlling the drivemechanism depending upon a piece of information (image informationformed on the CCD device, distance information of from an endoscope tipto a subject-of-observation). This is the same as for examples 2 to 4shown in the below.

Table 1 shows, in the upper thereof, the values of a radius of curvatureR of each lens, a center thickness of each lens, an air spacing Dbetween lenses (hereinafter, referred to as “surface-to-surface axialspacing”), a refractive index N_(d) at d-line of each lens, and an Abbenumber νd at the d-line of each lens, of the endoscopic objective lensaccording to example 1. Note that, in Table 1 and in the followingtables 2 to 4, the radius of curvature R and the surface-to-surfaceaxial spacing D are in values the focal length in the most-distantobservation state is normalized as 1.0 wherein the numbers putcorresponding to the symbols are provided increasing gradually away fromthe object.

In the lower of Table 1, there are shown a distance to the object(normalized similarly to the surface-to-surface axial spacing D, whichis true for those of the following Tables 2 to 4), a magnification and avariable group-to-group spacing 1 to 5 of surface-to-surface axialspacing D, in the states of most-distant observation, intermediateobservation and nearest observation in example 1. It is apparent fromthis that the endoscopic objective lens in the example 1 has a smallerchange of observing magnification upon focal adjustment in the nearsidemagnifying observation state, wherein focal adjustment is easy toperform.

TABLE 1 Radius-of- Refractive Abbe number curvature R Spacing D indexN_(d) νd  1 ∝ 0.3057 1.88300 40.9  2 0.8064 0.7644  3 −9.8211 0.26751.83400 37.2  4 1.2184 0.9019 1.48749 70.2  5 −1.4530 0.8079(variable 1)  6 2.8778 0.6497 1.57135 53.0  7 −1.9766 0.1911 (variable2)  8 ∝ 0.2293 1.88300 40.9  9 1.5249 0.0413 10 ∝ 0.6345 (Aperture stop)(variable 3) 11 1.5249 0.5733 1.58144 40.7 12 −9.4252 0.0764 13 1.70370.3057 1.83481 42.7 14 1.0563 0.5763 (variable 4) 15 1.2184 1.09301.48749 70.2 16 −1.0563 0.2675 1.84666 23.8 17 −4.8651 0.6879 (variable5) 18 ∝ 2.1402 1.55920 53.9 19 ∝ 0.0382 20 ∝ 0.2293 1.51633 64.1 21 ∝Most-distant Intermediate Nearest point point point Distance to object10.9302 3.2103 1.7580 Magnification −0.0864 −0.4054 −0.6816 Variable 10.8079 0.2405 0.2405 Variable 2 0.1911 1.1258 1.1258 Variable 3 0.63450.2672 0.2672 Variable 4 0.5763 0.5763 0.1758 Variable 5 0.6879 0.68791.0884

Example 2

FIG. 2 shows a basic arrangement of an endoscopic objective lensaccording to an example 2, in the states of most-distant observation,intermediate observation and nearest observation.

The endoscopic objective lens in example 2 includes, in order from theobject side, a first lens group G₁ having a negative refractive powerand being fixed, a second lens group G₂ having a positive refractivepower and movable along the optical axis, a third lens group G₃ having anegative refractive power and movable along the optical axis, a fourthlens group G₄ having a positive refractive power and being fixed, and afifth lens group G₅ having a positive refractive power and movable alongthe optical axis, similarly to those of the example 1.

The first, second and third lens groups G₁, G₂, G₃ are structuredsimilarly to those of the example 1. However, an aperture stop 1 isarranged between the second lens group G₂ and the third lens group G₃.Meanwhile, the present example is similar to the example 1 in that alens group C (G_(C)) is constituted by the first lens group G₁ while alens group A (G_(A)) is constituted by two lens groups of the second andthird lens groups G₂, G₃. The lens movement in the first focaladjustment is similar to that in the example 1. However, the aperturestop 1 is not to move during the first focal adjustment (not movedduring the second focal adjustment).

The fourth lens group G₄ is constituted by a sixth lens L₆ formed by adouble-convex lens, a seventh lens L₇ formed by a double-convex lenshaving an intense curvature in the image-side surface as compared to theobject-side surface, and an eighth lens L₈ formed by a double-concavelens having an intense curvature in the object-side surface as comparedto the image-side surface. The seventh lens L₇ and the eighth lens L₈are joined with each other.

The fifth lens group G₅ is structured as one cemented lens that is madeby joining together a ninth lens L₉ formed by a double-convex lens and atenth lens L₁₀ formed by a plane-concave lens whose concave surface isdirected toward the object side. Incidentally, the lens group B (G_(B))is constituted by only the fifth lens G₅, similarly to the example 1.The lens movement in the second focal adjustment is also similar to thatin the example 1.

Table 2 shows, in the upper thereof, the values of a radius of curvatureR of each lens, a surface-to-surface axial spacing D, a refractive indexN_(d) at the d-line of each lens, and an Abbe number ν_(d) at the d-lineof each lens, of the endoscopic objective lens according to the example2.

In the lower of Table 2, there are shown the values of a distance to theobject, a magnification and a variable group-to-group spacing 1 to 5 ofsurface-to-surface axial spacing D, in the states of most-distantobservation, intermediate observation and nearest observation in example2. It is apparent from this that the endoscopic objective lens in theexample 2 has a smaller change of observing magnification upon focaladjustment in the nearside magnifying observation state, and hence focaladjustment is easy to perform.

TABLE 2 Radius-of- Refractive Abbe number curvature R Spacing D indexN_(d) ν_(d)  1 ∝ 0.3024 1.88300 40.8  2 0.7806 1.0011  3 −4.0791 0.26461.88300 40.8  4 0.9499 0.7257 1.53172 48.9  5 −1.1705 0.7889(Variable 1)  6 4.5575 0.4914 1.48749 70.2  7 −1.4772 0.1888 (Variable2)  8 ∝ 0.1134 (Aperture stop) (variable 3)  9 ∝ 0.2268 1.88300 40.8 101.7636 1.0071 (variable 4) 11 1.7477 0.7560 1.438750 95 12 −2.41550.0756 13 2.8472 0.8996 1.67270 32.1 14 −1.0280 0.2646 1.88300 40.8 152.1495 0.5262 (variable 5) 16 1.6657 0.9903 1.60300 65.5 17 −1.66600.2646 1.84666 23.8 18 ∝ 0.6804 (variable 6) 19 ∝ 2.1167 1.55920 53.9 20∝ 0.0378 21 ∝ 0.2268 1.51633 64.1 22 ∝ Most-distant Intermediate Nearestpoint point point Distance to object 10.8102 3.02383 1.6253Magnification −0.0873 −0.4347 −0.7319 Variable 1 0.7889 0.4292 0.4292Variable 2 0.1888 0.5485 0.5485 Variable 3 0.1134 0.8137 0.8137 Variable4 1.0071 0.3068 0.3068 Variable 5 0.5262 0.5262 0.0782 Variable 6 0.68040.6804 1.1284

Example 3

An endoscopic objective lens according to example 3 is structuredsimilarly to that of the example 2 (however, the sixth lens L₆ thereofis made aspheric at both surface, as shown in the Table 3 given below).The lens movement in the first and second focal adjustment is similar tothat in the example 2. For this reason, there is omitted of showing thebasic arrangement of the endoscopic objective lens in the example 2, inthe states of most-distant observation, intermediate observation andnearest observation.

Table 3 shows, in the upper thereof, the values of a radius of curvatureR of each lens, a surface-to-surface axial spacing D, a refractive indexN_(d) at the d-line of each lens, and an Abbe number ν_(d) at the d-lineof each lens, of the endoscopic objective lens according to example 3.

In the middle of Table 3, there are shown the values of a distance tothe object, a magnification and a variable group-to-group spacing 1 to 6of surface-to-surface axial spacing D, in the states of most-distantobservation, intermediate observation and nearest observation in example3. It is apparent from this that the endoscopic objective lens inexample 3 has a smaller change of observing magnification upon focaladjustment in the nearside magnifying observation state, wherein focaladjustment is easy to perform.

Incidentally, in Table 3 and in the ensuing Table 4, the surface withthe mark “*” attached in the left of the surface number is made as anaspheric surface whose shape is defined by the followingaspheric-surface formula. In Table 3 and in the ensuing Table 4, theradius-of-curvature of the aspheric surface is represented as a value ofradius-of-curvature R on the optical axis in each of the tables. In thecounterpart lens arrangement figure, the extensions include those notnecessarily drawn from the intersection with the optical axis, for thesake of easier viewing of the figures.

In the lower of Table 3, there are shown the values of constants K, A₄,A₆ and A₈ corresponding to the spherical surfaces.

TABLE 3 Radius-of- Refractive Abbe number curvature R Spacing D indexN_(d) νd  1 ∝ 0.2989 1.88300 40.8  2 0.7807 1.0028  3 −4.1039 0.26151.88300 40.8  4 0.9127 0.7174 1.53172 48.9  5 −1.3021 0.7322(variable 1)  6 3.2132 0.4857 1.48749 70.2  7 −1.4993 0.1867 (variable2)  8 ∝ 0.1121 (Aperture stop) (variable 3)  9 ∝ 0.2242 1.88300 40.8  101.8768 1.0764 (variable 4) *11 1.7583 0.7473 1.43425 95 *12 −2.43480.0747  13 2.7944 0.8892 1.67270 32.1  14 −0.9632 0.2615 1.88300 40.8 15 2.0152 0.5203 (variable 5)  16 1.6057 0.9789 1.60300 65.5  17−1.5765 0.2615 1.84666 23.8  18 −2687.0228 0.6725 (variable 6)  19 ∝2.0923 1.55920 53.9  20 ∝ 0.0375  21 ∝ 0.2242 1.51633 64.1  22 ∝Most-distant Intermediate Nearest point point point Distance to object10.6858 2.9890 1.6066 Magnification −0.0884 −0.4342 −0.7334 Variable 10.7322 0.4382 0.4382 Variable 2 0.1867 0.4807 0.4807 Variable 3 0.11210.8827 0.8827 Variable 4 1.0764 0.3058 0.3058 Variable 5 0.5203 0.52030.0776 Variable 6 0.6725 0.6725 1.1152 Aspheric surface coefficient K A₄A₆ A8 11 0.9998 −6.9088 × 10⁻² 6.6822 × 10⁻² −1.1525 × 10⁻¹ 12 1.0001−6.7675 × 10⁻² 3.8385 × 10⁻² −1.0546 × 10⁻¹ *Aspheric surfaceAspheric-Surface Formula:

$Z = {\frac{Y^{2}/R}{1 + \sqrt{1 - {K \times {Y^{2}/R^{2}}}}} + {\sum\limits_{i = 2}^{4}{A_{2i}Y^{2i}}}}$where

Z: a length of a vertical line drawn from a point on an aspheric surfacedistant Y from the optical axis onto a tangential plane at the asphericpoint (plane vertical to the optical axis),

Y: a distance from the optical axis

R: a radius of curvature of the aspheric surface at a point close to theoptical axis,

K: the eccentricity,

A_(2i): an aspheric surface coefficient (i=2-4)

Example 4

FIG. 3 shows a basic arrangement of an endoscopic objective lensaccording to an example 4, in the states of most-distant observation,intermediate observation and nearest observation.

The endoscopic objective lens in example 4 includes, in order from theobject side, a first lens group G₁ having a negative refractive powerand being fixed, a second lens group G₂ having a positive refractivepower and being fixed, a third lens group G₃ having a negativerefractive power and movable along the optical axis, a fourth lens groupG₄ having a positive refractive power and being fixed, and a fifth lensgroup G₅ having a positive refractive power and movable along theoptical axis, as explained in the second embodiment.

The first lens group G₁ is constituted by a first lens L₁₀ formed by aplane-concave lens whose concave plane is directed toward the imageside, a second lens L₂ formed by a negative meniscus lens whose concavesurface is directed toward the image side, and a third lens L₃ formed bya double-convex lens. The second lens L₂ and the third lens L₃ arejoined with each other.

The second lens group G₂ is constituted by only the fourth lens L₄formed by a double-convex lens having an intense curvature in theimage-side surface as compared to that of the object-side surface. Thethird lens group G₃ is constituted by only the fifth lens L₅ formed by aplane-concave lens whose concave surface is directed toward the image.Meanwhile, as explained in the second embodiment, in the example 4, alens group C (G_(C)) is constituted by the first and second lens groupsG₁, G₂, while a lens group A (G_(A)) is constituted by only the thirdlens group G₃. The first focal adjustment, in a distant observationstate, is performed by moving the third lens group G₃ (fifth lens L₅)along the optical axis toward the image from a position in themost-distant observation state shown in the upper in FIG. 3 (no movementis made in the second focal adjustment). As explained in the secondembodiment, in the example 4, a fixed aperture stop 1 is arrangedbetween the second lens group G₂ and the third lens group G₃.

The fourth lens group G₄ is constituted by a sixth lens L₆ made asphericat both surfaces and having a positive refractive power, a seventh lensL₇ formed by a double-convex lens having an intense curvature in theimage-side surface as compared to that of the object-side surface, andan eighth lens L₈ formed by a double-concave lens having an intensecurvature in the object-side surface as compared to that of theimage-side surface. The seventh lens L₇ and the eighth lens L₈ arejoined with each other.

The fifth lens group G₅ is constituted by one cemented lens that joinedtogether are a ninth lens L₉ formed by a double-convex lens and a tenthlens L₁₀ formed by a plane-concave lens whose concave surface isdirected toward the object. As explained in the second embodiment, inthe example 4, a lens group B (G_(B)) is constituted by only the fifthlens group G₅. The second focal adjustment is performed by moving thefifth lens group G₅ along the optical axis toward the object side from aposition in the intermediate observation state shown in the middle inFIG. 3 (no movement is made during the first focal adjustment).

Table 4 shows, in the upper thereof, the values of a radius of curvatureR of each lens, a surface-to-surface axial spacing D, a refractive indexN_(d) at the d-line of each lens, and an Abbe number ν_(d) at the d-lineof each lens, of the endoscopic objective lens according to the example4.

In the middle of Table 4, there are shown the values of a distance tothe object, a magnification and a variable group-to-group spacing 1 to 4of surface-to-surface axial spacing D, in the states of most-distantobservation, intermediate observation and nearest observation in example4. It is apparent from this that the endoscopic objective lens inexample 4 has a smaller change of observing magnification upon focaladjustment in the nearside magnifying observation state, wherein focaladjustment is easy to perform.

Furthermore, in the lower of Table 4, there are shown the values ofconstants K, A₄, A₆ and A₈ corresponding to the spherical surfaces.

TABLE 4 Radius-of- Refractive Abbe number curvature R Spacing D indexN_(d) ν_(d)  1 ∝ 0.2725 1.88300 40.8  2 0.6916 0.9810  3 4.3863 0.23851.83481 42.7  4 0.8598 0.6541 1.48749 70.2  5 −1.8000 0.6675  6 5.26870.4429 1.48749 70.2  7 −1.3362 0.1703  8 ∝ 0.1022 (Aperture stop)(variable 1)  9 ∝ 0.2044 1.88300 40.8  10 2.0063 1.1709 (variable 2) *111.3925 0.6813 1.43425 95 *12 26.1064 0.0681  13 1.6512 0.8108 1.6398034.5  14 −0.8347 0.2385 1.88300 40.8  15 2.3662 0.4612 (variable 3)  161.3748 0.8926 1.56907 71.3  17 −1.4637 0.2385 1.84666 23.8  18 ∝ 0.6132(variable 4)  19 ∝ 1.9077 1.55920 53.9  20 ∝ 0.0341  21 ∝ 0.2044 1.5163364.1  22 ∝ Most-distant Intermediate Nearest point point point Distanceto object 9.7431 2.7253 1.4649 Magnification −0.0970 −0.4262 −0.7115Variable 1 0.1022 0.9801 0.9801 Variable 2 1.1709 0.2930 0.2930 Variable3 0.4612 0.4612 0.0706 Variable 4 0.6132 0.6132 1.0038 Aspheric surfacecoefficient K A₄ A₆ A₈ 11 0.9983 −3.9683 × 10⁻² 1.8236 × 10⁻¹ −6.4698 ×10⁻² 12 1.0001 −3.4127 × 10⁻² 1.1702 × 10⁻¹ −1.0082 × 10⁻¹ *asphericsurface

Table 5 shows the values corresponding to the conditional expressions(1) to (5) for examples 1 to 4. Examples 1 to 4 each satisfy all thecorresponding conditional expressions (1) to (5).

TABLE 5 Example 1 Example 2 Example 3 Example 4 Conditional 1.47 1.481.46 1.32 expression (1), (3) Conditional 0.98 0.96 0.96 0.97 expression(2) Conditional 7.69 8.89 8.83 8.30 expression (4) Conditional 4.18 4.444.51 5.34 expression (5)

FIGS. 4 to 7 show aberrations (spherical aberration, astigmatism,distortion and lateral color) on examples 1 to 4, in the most-distantobservation, intermediate observation and nearest observation states. Inthe aberration diagram, ω represents a half angle of view. As shown inFIGS. 4 to 7, the foregoing aberrations can be all provided preferableby each of examples 1 to 4.

Incidentally, the endoscopic objective lens in the invention is notlimited to the foregoing examples but can be changed to variousmodifications. For embodiment, radius-of-curvature R andsurface-to-surface axial spacing D can be suitably changed for thelenses.

Meanwhile, the endoscopic objective lens in the embodiment wasstructured not overlapped between the lenses making up the lens group A(G_(A)) and the lenses making up the lens group B (G_(B)). However, thelenses, making up the lens group A (G_(A)), can be provided constitutinga part or the whole of the lens group B (G_(B)) or the lenses, making upthe lens group B (G_(B)), can be provided constituting a part or thewhole of the lens group A (G_(A)).

Meanwhile, aspheric surfaces, GRIN lenses or diffraction gratings can beadded to or substituted in the endoscopic objective lens in each of theexamples, thereby correcting for chromatic aberration or otheraberrations.

While the invention has been described with reference to the exemplaryembodiments, the technical scope of the invention is not restricted tothe description of the exemplary embodiments. It is apparent to theskilled in the art that various changes or improvements can be made. Itis apparent from the description of claims that the changed or improvedconfigurations can also be included in the technical scope of theinvention.

This application claims foreign priority from Japanese PatentApplication No. 2005-347817, filed Dec. 1, 2005, the entire disclosureof which is herein incorporated by reference.

1. An objective lens for an endoscope, comprising a lens group A and alens group B that are movable in a direction of an optical axis, whereinfirst focal adjustment for observing a point between a most-distantpoint and an intermediate point is performed by moving only the lensgroup A from a lens arrangement for observing the most-distant point;second focal adjustment for observing a point between the intermediatepoint to a nearest point is performed by moving only the lens group Bfrom a lens arrangement for observing the intermediate point; a lensconstituting the lens group A and a lens constituting the lens group Bdo not overlap each other; and the lens group B consists of a cementedlens.
 2. An objective lens for an endoscope, comprising a lens group Aand a lens group B that are movable in a direction of an optical axis,wherein first focal adjustment for observing a point between amost-distant point and an intermediate point is performed by moving onlythe lens group A from a lens arrangement for observing the most-distantpoint; second focal adjustment for observing a point between theintermediate point to a nearest point is performed by moving only thelens group B from a lens arrangement for observing the intermediatepoint; a lens constituting the lens group A and a lens constituting thelens group B do not overlap each other; and the lens group A comprisestwo groups that are a lens group having a positive refractive power anda lens group having a negative refractive power, the two groups beingarranged to take respective moving paths different from each otherduring the first focal adjustment.
 3. An objective lens for anendoscope, comprising a lens group A and a lens group B that are movablein a direction of an optical axis, wherein first focal adjustment forobserving a point between a most-distant point and an intermediate pointis performed by moving only the lens group A from a lens arrangement forobserving the most-distant point; second focal adjustment for observinga point between the intermediate point to a nearest point is performedby moving only the lens group B from a lens arrangement for observingthe intermediate point; a lens constituting the lens group A and a lensconstituting the lens group B do not overlap each other; the lens groupB consists of a cemented lens; and the lens group A comprises two groupsthat are a lens group having a positive refractive power and a lensgroup having a negative refractive power, the two groups being arrangedto take respective moving paths different from each other during thefirst focal adjustment.